Mainhart had been an officer in Yell County for five years.
According to the Arkansas State Police, there is an active manhunt underway in Yell County near Dardanelle.
My thoughts and prayers are with friends and family of Lt. Mainhart and the other victims of the tragedy in Yell County today.
The deputy was found by a passerby, who notified police.
An Arkansas man faces three counts of capital murder in the killing of three people, including a sheriff's deputy who was fatally shot during a traffic stop.
Authorities say the suspect was holed up in that home before he surrendered.
"From the moment he arrived, he was a leader not only in the department but in the community of Yell County, as well", Sheriff Bill Gilkey said. The suspect's father said the woman, who was apparently being held hostage, is Bowden's girlfriend.
Mainhart was fatally shot on State Highway 27 near Slo Fork Road, Sadler said.
Two bodies were found at the Gum Springs Road home.
"This deputy checked in for duty this morning not even realizing that this would be his last call", said Bill Sadler with the Arkansas State Police.
The deputy, whose name wasn't immediately released, was on his way to check a disturbance call on rural Gum Springs Road when he pulled someone over at a nearby intersection shortly after 7:15 a.m. CT, Sadler said.
The fatal shooting "highlights the dangers that are facing law enforcement officers across this country, " Sadler said.
Following news of Mainhart's death, Gov. Asa Hutchinson issued a statement expressing his "deepest sympathies" for the deputy's family.
Mainhart's death came on the same day as the 36th annual memorial to honor fallen officers at the Arkansas state Capitol.
He's also worked for Arkansas Tech and as security police in the U.S. Air Force.